When a player or team scores a point when the game is in a deuce, that player or team is said to have the advantage. For example, if Player A scores the point during the deuce, and Player B is serving, the announcer may say: "Player B to serve, advantage Player A" or "Player A's ad." Both these phrases would signify that Player A has the advantage, so Player A can win the game on this point.
In tennis, there is no difference in meaning between the terms ad and advantage. In fact, ad is just a short form of the word advantage. Moreover, both terms are used interchangeably to signify the same thing. In a tennis match, if both players or teams are tied at the end of a game (deuce), or a 40-40 deadlock, then the game is decided by whoever scores when holding the ad or advantage.
When a player or team has the advantage, then they have the opportunity to win the game during the next point. If a player or team has the ad and wins the point, then they win that game. However, if the player or team has the ad and loses the point, then the game goes into a deuce again and neither player or team has the advantage. In fact, the player or team that wins the next point then claims the advantage going into the following serve, giving that player or team the chance to win the game.